Use of percutaneous transpedicular external fixation pins to measure intervertebral motion.

Abstract

Direct measurement of intervertebral motion was compared to motion determined by measuring the position of the exposed ends of the external fixation pins. To verify the accuracy of this technique, so that this protocol can be used to study intervertebral motion in the clinical setting. The transpedicular external fixation test has been shown to be a test that can predict the outcome of spinal fusion. In patients who are candidates for this test, intervertebral motion can be calculated from motion at the external ends of these pins. Six fresh cadaveric spinal segments from L2 to L5 were instrumented with titanium Schanz screws. Reflective markers were placed on the tips of the pins, and intervertebral motion was measured using a noncontacting camera system. Computed tomography data were used to determine the position of the vertebra relative to the reflective markers. Intervertebral distances were calculated and compared with direct measurements obtained using a three-dimensional digitizing arm. There was an excellent correlation (r2 = 0.931) between the directly measured intervertebral motions and those that were indirectly calculated from measurements of motion at the end of the Schanz screws. Intervertebral motion can be measured by monitoring motion of the ends of transpedicular external fixation pins. Motion of anatomic landmarks on the vertebrae can be calculated from the pin end's motion if computed tomography data are used to determine the geometric relation between the vertebrae and the external fixation pins. This validation study supports the use of this method in clinical investigations of intervertebral motion in patients with low back pain and external fixation.